1. Field of the Invention
The present invention relates to a stacked dielectric band-pass filter that enables a passband to be wider than ever before.
2. Description of the Related Technology
In a high frequency circuit section of a radio communication device or the like, a band-pass filter is used for attenuating an unnecessary component such as harmonic waves. As this type of band-pass filter, a filter using a dielectric resonator, which is small and provides a good attenuation characteristic, is currently mainstream. In particular, as a resonator formed in a dielectric, a distributed constant filter using a strip line is widely used.
Recently, as seen in a UWB (Ultra Wide Band) system, a filter is required to be usable for a wide band in which a specific bandwidth (passband with/central frequency) is approximately 100% or more.
In a usual design method of a band-pass filter, the resonator was necessary to be multi-stepped to widen a passband. However, when the number of steps of the resonator is increased in this way, another difficulty occurs along with this, that is, increase in size and increase in insertion loss occur. In particular, when an ultra wide band as in the UWB system is achieved, since a resonator having only two to three steps cannot provide a necessary bandwidth, a filter that is small and low in loss has been hardly provided in the usual method.
Thus, as shown in JP-A-2006-101500, as a filter being effective for achieving a wider bandwidth while meeting the requirements of small size and low loss, a filter is proposed, which uses a fundamental band of a λ wave given by a λ/4 resonator, a third harmonic wave of the λ wave shifted to a lower band side, and a passband given by a λ/2 resonator for interpolation between the fundamental wave and the third harmonic wave, and thereby provides a flat pass characteristic in a bandwidth from the fundamental wave to the third harmonic wave.
To achieve a further wide band in such a filter, it is considered that the fundamental wave is shifted to a lower band side to widen a low frequency band, and a shift level of the third harmonic wave is reduced to widen a band at a higher band side. However, this makes it difficult to perform interpolation between the fundamental wave and the third harmonic wave using the passband given by the λ/2 resonator, as a result, a region being insufficiently interpolated may be formed. Consequently, there has been a problem that the flat pass characteristic can be currently obtained only in a band from 3 to 8 GHz.